The present invention relates to an apparatus for simulating electric-static discharges artificially. The present invention relates more specifically to a static-electricity generation simulator used to test and evaluate sensitivities and immunity characteristics of electronic assemblies to electrical overstress or electromagnetic interference caused by electrostatic discharges. The term "electronic assemblies" includes large scale integration circuitry (LSI), standard integrated circuitry (IC), miniaturized electronic parts, and various other electronic equipments.
Electrostatic discharges in the various kinds of electronic components and electronic apparata described above are mainly caused by electrified objects isolated from the earth, especially, among other things, by charged human bodies. Hence, most of electrostatic discharge simulators are modeled as equivalents to charged human bodies.
FIG. 1 shows the lay-out of a conventional electrostatic discharge simulator circuit. The conventional simulator shown in FIG. 1 is disclosed in a publication document by IEC (International Electrotechnical Committee) publication No. 801.2 in 1984 (Page 24).
In FIG. 1, the symbols C.sub.CD and R.sub.D denote a charge-and-discharge capacitor having an electrostatic capacity equivalent to that of a human body and a discharge resistor having a resistance value equivalent to that of the human body, respectively. The symbol SED denotes a charged electrode which usually comprises a ball-shaped or needle-tip-shaped (tapered) electrode. The symbol GAP denotes a discharge gap and TO denotes a specimen (object to be tested) such as an electronic component. As the discharge electrode SED gradually approaches the specimen TO or directly touches the specimen while the charge-and-discharge capacitor C.sub.CD is charged to a predetermined voltage V.sub.o, the electric charge on the charge-and-discharge capacitor C.sub.CD is instantaneously transferred to the specimen TO by a spark discharge generated when the product of the size of the discharge gap GAP and atmospheric pressure reaches a value according to the charged voltage of the charge-and-discharge capacitor C.sub.CD or by electrical contact.
Since the voltage across the human body can often reach several kilovolts to approximately 15 through 16 kilovolts and in special cases it reaches a maximum a discharge start voltage of approximately 35 through 36 kilovolts, the voltage V.sub.o across the charge-and-discharge capacitor C.sub.CD can be set over a voltage range from zero volts to approximately 35 through 36 kilovolts, which clearly accords with the voltages of the electrified human body. Although the equivalent resistance value of the discharge resistor R.sub.D and electrostatic capacitance value of the charge-and-discharge capacitor C.sub.CD are also determined according to the actual values of the charged human body, these values change over a considerably wide range. However, in this example, the equivalent electrostatic capacitance of the charge-and-discharge capacitor C.sub.CD has a fixed value appropriately selected from the range of 50 picofarads to 250 picofarads and the equivalent resistance of the discharge resistor R.sub.D has a fixed value appropriately selected from the range of 100 ohms to 1500 ohms.
The discharge current of the conventional electrostatic discharge simulating apparatus shown in FIG. 1 will be described below.
The waveform of the discharge current can be principally expressed as an exponential curve as shown in FIG. 2 if the impedance of the specimen TO is negligible. In FIG. 2, the horizontal axis denotes elapsed time t, the vertical axis denotes a current i, t.sub.o denotes the time at which current discharge starts and ##EQU1## denotes the peak current, which appears at the time t.sub.o of discharge start. A discharge time constant .tau. can be expressed as follows: ##EQU2##
On the other hand, the discharge current i can be expressed as follows: ##EQU3## wherein C.sub.CD denotes the electrostatic capacitance of the charge-and-discharge capacitor C.sub.CD and R.sub.D denotes the resistance of the discharge resistor R.sub.D.
However, the actual waveform of the discharge current differs from the theoretical waveform shown in FIG. 2 due to some factors specific to spark discharge phenomena. FIGS. 3(a) through 3(d) illustrate actual waveforms of the discharge current described above (the horizontal and vertical axes are as shown in FIG. 2). It should be noted that in FIGS. 3(a) through 3(d) the electrostatic capacitance of the charge-and-discharge capacitor C.sub.CED is a constant value, i.e., 120 picofarads, the resistance of the discharge resistor R.sub.D is a constant value, i.e., 250 ohms, and the charge voltage V.sub.o across the charge-and-discharge capacitor C.sub.D (the discharge start voltage at the discharge gap GAP, i.e., the same as the breakdown voltage of air across the discharge gap) is V.sub.o =0.5 kilovolts in FIG. 3(a), V.sub.o =3 kilovolts in FIG. 3(b), V.sub.o =7 kilovolts in FIG. 3(c), and V.sub.o =15 kilovolts in FIG. 3(d). As appreciated from FIGS. 3(a) through 3(d), the waveform of discharge current varies as the voltage V.sub.o varies.
The pattern of change in the waveform of the discharge current also appears in observations on the actual waveforms of the discharge current generated during electrostatic discharges from the human body. The causes of these aberration have not been investigated previously because they so closely resemble the characteristics of electrostatic discharge from human bodies.
However, repeated experiments by the Applicant using the electrostatic discharge simulator have revealed that changes in the waveform of the discharge current depend not only upon the discharge start voltage across the discharge gap (voltage across the charge-and-discharge current) but also upon the following factor.
The maximum value I.sub.p of the discharge current is not always proportional to the discharge start voltage V.sub.o, as denoted by a first curve a in FIG. 4, (it should be noted that in FIG. 4, the horizontal axis denotesthe discharge start voltage V.sub.o and the vertical axis denotes the maximum value I.sub.p of the discharge current). In fact, the rate of increase of the maximum value I.sub.p tends to decrease as the discharge start voltage V.sub.o increases. The other curve b in FIG. 4 denots the theoretical value I.sub.p (=V.sub.o /R.sub.o).
In addition, the rise time of the discharge current increases as the discharge start voltage V.sub.o increases, as shown in FIGS. 5(a) through 5(c).
In FIGS. 5(a) through 5(c), the horizontal axis denotes elapsed time and the vertical axis denotes the instantaneous value i of the discharge current. FIG. 5(a) shows the current i when the discharge start voltage V.sub.o is relatively low, FIG. 5(b) shows the current value i when the discharge start voltage V.sub.o is intermediate, and FIG. 5(c) shows the current value i when the discharge start voltage V.sub.o is relatively high. It should be noted that in FIGS. 5(b) and 5(c), the symbol t.sub.r denotes the time required for the current i to reach a magnitude of 90% of peak value from the magnitude of 10%. As appreciated from FIGS. 5(a) through 5(c), as the discharge start voltage V.sub.o increases the rise time of the discharge current tends to increase. In addition, although not shown in FIGS. 5(a) through 5(c), the actual waveforms of the discharge current will exhibit defects such as shown in FIGS. 3(b) through 3(d), i.e., will not conform to the ideal waveform (denoted in dotted lines in FIGS. 3(b) through 3(d)) of the discharge current.
This phenomenon may be due to the following factors. As the discharge start voltage V.sub.o increases, the size of the discharge gap GAP increases. As the size of the discharge gap GAP increases, the electrical field strength across the discharge gap GAP decreases.
FIG. 6 shows an example of the electric field intensity E with respect to the size d of the discharge gap. As shown in FIG. 6, the electric field intensity E remains essentially constant after the size of the discharge gap GAP reaches a certain value.
The increase in the discharge start voltage V.sub.o has a given relationship with the increase in the size of the discharge gap GAP. In addition, the spark discharge phenomenon is related to dielectric breakdown of gas (in this case, air), i.e., a transient phenomenon involving a rapid and radical state change from an insulating state to a conducting state.
Supposing that the above-described transient state change can be considered to be a process of increasing the conductivity within a spark spacing, the speed of the increase in conductivity is determined by the size of the discharge gap and the spatial electric field intensity across the discharge gap under conditions of constant air pressure. Hence, as shown in FIG. 7, as the length of the discharge gap GAP increases in accordance with the increase in the discharge start voltage V.sub.o applied between a fixed discharge electrode SED.sub.F and a movable discharge electrode SED.sub.M, the time required for the increase in conductivity .rho.(t) of the spark spacing also increases.
As shown in FIG. 8 (the horizontal axis denotes elapsed time t and the vertical axis denotes voltage V and current i), when the discharge is initiated at the time t=t.sub.o, part of the electric charge on the charge-and-discharge capacitor C.sub.CD is dissipated at the spark spacing as heat loss expressed by i.sup.2.1/.rho.(t) and is discharged and dissipated into the specimen between the start of discharge and the arrival of the peak value I.sub.p of the discharge current i. Hence, the voltage across the charge-and-discharge capacitor C.sub.CD drops below the voltage V.sub.o at the time t.sub.1. If the amount of electric charge dissipated at the specimen is q, the reduced voltage v can be expressed as: ##EQU4##
If V.sub.o -v=V.sub.1, the discharge current i at that time is expressed as follows: ##EQU5##
Since at this time the conductivity at the spark spacing will not be at its maximum value, the maximum value I.sub.p of the discharge current (bold solid line in FIG. 8) at this time can be expressed as: ##EQU6## Consequently, the maximum value I.sub.p will be considerably lower than V.sub.o /R.sub.D. It should be noted that the thin solid line in FIG. 8 denotes the ideal waveform of the discharge current and the dotted line denotes the voltage waveform of the charge-and-discharge capacitor C.sub.CD.
The reasons for the various defects in the waveform of the discharge current, i.e. the aberrations form the ideal waveform of the discharge current as shown in FIGS. 3(b) through 3(d)) can be estimated as given below.
Part of the electric charge on the charge-and-discharge capacitor C.sub.CD during the increase in conductivity at the spark spacing is dissipated at the spark spacing so that the electric field intensity at the spark spacing during generation of the spark discharge is reduced. The increase of the conductivity is correspondingly suppressed due to the reduction in the electric field intensity. Thus, the conductivity will not conform to the normal increase curve with respect to time but rather assumes the shape of a mesa or other form in which the rate of increase first gradually increases with respect to time and then the conductivity finally decreases after a slight delay.
For this reason, the rise time of the discharge current may lag remarkably behind the theoretical rise time determined by the size d and air pressure P of the discharge gap GAP.
The conventional electrostatic simulator which applies an electric charge to the specimen by way of a spark discharge comprises, as shown in FIG. 9, a variable power supply (battery) V.sub.s having a rated capacity of 0 volts through kilovolts in the thirties, a charge resistor R.sub.c, a switch SW.sub.c, a charge-and-discharge capacitor C.sub.CD having an electrostatic capacity from 50 picofarads to 250 picofarads, a discharge resistor R.sub.D having a resistance value from 100 ohms to 1500 ohms, and discharge electrode SED. The equivalent circuit for the main part of the circuit shown in FIG. 9 is shown in FIG. 10. Stray capacitance C.sub.g having a value from one picofarad to several picofarads is present across the discharge gap GAP between the discharge electrode SED and the opposing electrode SED.sub.TO, which is actually part of the specimen TO. The magnitude of the stray capacitance C.sub.g is determined by the surface areas of the opposing electrodes SED and SED.sub.TO, provided that the dielectricity of air in the discharge gap GAP is constant, and varies according to the size of the discharge gap GAP between the electrode SED and the opposite electrode SED.sub.TO.
Although the presence of the stray capacitance C.sub.g can be neglected at the initial stage of spark discharge since the electric charge in the stray capacitance C.sub.g can quickly dissipate at the spark spacing, the following may occur if the stray capacitance C.sub.g is intentionally increased.
Although at the initial stage of spark discharge across the discharge gap GAP the electric charge across the charge-and-discharge capacitor C.sub.CD is sent into the spark spacing via the discharge resistor R.sub.D, the amount of charge reaching the spark spacing will be extremely small due to the limitations set by the spark resistance 1/.rho.(t) across the discharge gap GAP and discharge resistor R.sub.D.
However, since the other electric charge across the stray capacitance C.sub.g is limited only by the spark resistance 1/.rho.(t) across the discharge gap GAP irrespectively of the discharge resistor R.sub.D, the charge transfer will be extremely large compared with the amount of electric charge from the charge-and-discharge capacitor C.sub.CD. After the conductivity .rho.(t) across the discharge gap GAP increases, all of the electric charge across the stray capacitance C.sub.g is sent solely to the spark spacing. When the impedance of the current path connecting the stray capacitance C.sub.g and the discharge gap GAP is reduced, the speed at which the electric charge across the stray capacitance C.sub.g enters the spark spacing can be increased. Consequently, the arc can cross the discharge gap GAP with little loss of the electric charge from the charge-and-discharge capacitor C.sub.CD.
The output energy in the electrostatic discharge simulator is denoted by i in FIG. 10 and is determined by the current through the discharge gap GAP from the charge-and-discharge capacitor C.sub.CD, and is independent of the electric charge within the stray capacitance C.sub.g.